The present invention broadly relates to automatic welding of a square steel column having a large cross section and, more particularly, to a self-propelling apparatus for automatic welding which runs along the surface of the column so as to secure a constant welding speed over the entire welding line including straight side portions and radiused corner portions (referred to as "R portion", hereinafter).
Construction of a building with steel reinforcements with square steel columns (referred to as "square column", hereinafter) generally requires horizontal position welding of the square columns. Such welding is usually conducted in the form of multi-layers welding, using a semi-automatic welding apparatus of gas shielded metal arc type or self-shielded arc type.
However, the use of such a semi-automatic welding apparatus requires workers having high levels of skill and experience. In recent years, a serious problem has been posed because there is a shortage of such skilled workers and because existing skilled workers are getting old. Under this circumstance, there has been an increasing demand for full automation of welding of the kind described.
A known automatic welding system employed in this type of welding is aided by a computer which controls the welding speed such that a constant welding speed is maintained over the entire welding path including straight and R portions of the square column.
This known automatic welding system is very large in size and is costly. In addition, an impractically long time is required for training the operators who will use this welding system. This automatic welding system, therefore, could hardly be put to practical use.
In order to obviate this problem, the present applicant filed a patent application on an invention under the title of "WELDING SPEED CONTROL DEVICE FOR WELDING RADIUSED CORNER PORTION OF LARGE-SIZED SQUARE STEEL COLUMN", on the date of Jul. 22, 1991, as Japanese Patent Application No. 3-271738.
The welding speed control device of the above-mentioned patent application has a combination of a limit switch installed on a truck or a carriage which runs along a rail and a limit switch actuator which is installed at a preset position along the trail.
More specifically, the welding speed control device has a limit switch actuator installed at a position along the rail and a limit switch attached to a carriage, wherein the limit switch actuator and the limit switch cooperate with each other so as to enable controlling of the welding speed such way that the welding is conducted at a speed V.sub.1 in the region between a point A and a point B and, in the region between a point D and a point E, at a speed V.sub.2 higher than the speed V.sub.1, then the welding speed being then lowered again to V.sub.1 as the welding passes the point E. Consequently, horizontal weld joint of a square column can be made smoothly and continuously over the entire seam path, i.e., from a straight portion to an R portion and then to the next straight portion.
This welding speed control device, however, has disadvantages in that the device itself is expensive and is rather difficult to operate.